The thieves broke down the door and through the terrarium, and even into the private offices of the zoo, in what Hafström believes was an attempt to find more snakes to pinch.

"We had 15 that were born here in July, and we only left two out on show. They must have been looking for the other 13," he said. "It's extremely rare that pit vipers are born in captivity."

Hafström hopes the media attention will mean the thieves have trouble selling the pair of pit vipers. He added that the public has no need for concern, even though the snakes are highly venomous.

"There's no way they'll let the snakes loose, they're too valuable. And even if they did, the snakes wouldn't survive the cold here. No one has anything to worry about."

The police have launched an investigation.

The pit viper, also known as the Trimeresurus, is nicknamed the 100 pace snake as those bitten by it are said to only manage 100 steps before dropping dead. It is mostly found in the wild in southern Asia, and eats rodents and birds.